Fingerprint analysis is amongst the most widely used and studied biometric techniques. During the last two decades, many new and exciting developments have taken place in the field of fingerprint science, summarized for example in the monograph Advances in Fingerprint Technology, 2nd ed., edited by H. C. Lee and R. E. Gaensslen (CRC Press, 2001). Fingerprint identification not only plays a major role in forensic or police science, but also in controlling the building-access or information-access of individuals to buildings, rooms, and devices such as computer terminals.
When facing the task of actually imaging a given fingerprint, a problem associated with fingerprint imagers concerns the reliable and accurate transformation of the ridge and trough pattern of the fingertip into electrical or optical signals to be stored in a digital format. To achieve this task, optical systems are well known in the art, and were described for example in U.S. Pat. No. 5,109,427 to Yang issued Apr. 28, 1992, in U.S. Pat. No. 5,187,748 to Lee, issued Feb. 16, 1993, and in U.S. Pat. No. 5,233,404 to Lougheed et al. issued Aug. 3, 1993. However, such devices require sophisticated equipment and tend to be bulky and costly.
In an attempt to overcome some of the limitations and disadvantages associated with optical systems based on illumination of the fingertip, in U.S. Pat. No. 4,353,056 to Tsikos issued Oct. 5, 1982, an alternative kind of fingerprint imager that uses a capacitive sensing approach, is disclosed. The described sensor has a two dimensional array of capacitors, each of which comprises a pair of spaced electrodes, carried in a sensing member and covered by an insulating film. The sensors rely upon deformation to the sensing member caused by a finger being placed thereon, so as to locally vary the spacing between capacitor electrodes, and hence the capacitance of the capacitors, according to the ridge and trough pattern of the fingerprint. Although this device may not suffer from the problems associated with the kind of sensor employing an optical sensing technique, it suffers from its own problems. The difficulties that arise are mainly related to circuitry, the need of capacitor charging, and component connections. Thus, in another attempt to improve upon deficiencies and limitations of the aforementioned and other prior art, a further contact-imaging device is described in U.S. Pat. No. 5,325,442 to Knapp issued Jun. 28, 1994.
Knapp teaches a capacitance measuring imaging device in the form of a single large active matrix array, using deposition and definition by photolithographic processes of a number of layers on a single large insulating substrate. Although Knapp's attempt provides an improvement over Tsikos' device mentioned above, other disadvantages and limitations become evident in the implementation of a manufacturing process. Such a process relies on a single large imaging contact device, which often is a silicon die cut from a silicon wafer. Since a contact area of approximately 5 cm 2 is needed for imaging a fingerprint, the silicon devices are costly to manufacture, they have a low manufacturing yield, they are fragile, and thus have limited use in contact applications. These fragility and cost limits prohibit the widespread use of capacitive imaging of fingerprints.
Reducing the size of the imaging array is beneficial to limit cost and production yield limitations. That said, an imager with significantly reduced size is likely to be even more fragile than a larger imager. In U.S. patent application Ser. No. 09/984,354 filed on Oct. 30, 2001, an imaging system is described that includes a small capacitive array of sense elements in conjunction with a processor for forming a composite image of a fingerprint from a plurality of images of small sections of a single fingertip captured, for example, during a swiping motion. The sensing device is disposed between protective surfaces and supported by a flexible structure.
Unfortunately, due to friction between a fingertip and a swipe fingerprint sensor, the fingerprint imaged thereby is deformed. Different amounts of pressure, different dryness or moisture levels, different swipe speeds, and different swipe directions result in very different raw image data. Of course, for best results in fingerprint analysis, similar images are preferred and as such, reproducible imaging would be highly desirable. Unfortunately, this desire to have reproducible imaging frustrates widespread implementation of swipe fingerprint scanners such as the one mentioned above.
To make even better practical use of swipe fingerprint scanners, it would be advantageous to achieve enhanced control over the movement of the biological surface to be sensed relative to the swipe scanner surface, in at least one of space or time. A precise and well-controlled relative motion is particularly desirable in situations where the imager is operated in an environment with a high level of vibrational disturbance. It would be further advantageous to provide a device that reduces mechanical stress applied to the image-sensing device, thus increasing the robustness of the swipe imager.